Ask HN: How do phones and tablets have such high resolutions for cheap?

Why is it that I can buy a replacement LCD for an iPhone or iPad, or even just buy a phone with a super high-resolution screen for pretty cheap but can't find a standalone LCD with that kind of resolution?

I have found some weird HDMI-MIPI converters online, but most are still really limited in resolution and performance.

A 7 inch touch-screen retina to hook up to my Raspberry-PI would make for some really cool side projects.

Note: I know of solutions to hack lcd's but how come nobody comes out with standalone purchasable high-res small screens?